Superhydrophobic compositions and coating process for the internal surface of tubular structures

ABSTRACT

A method for depositing a coating includes creating a vacuum within an interior volume of a tubular structure, wherein the tubular structure also includes an internal surface. Gas is supplied to the interior volume of the tubular structure, wherein the gas includes a plasma precursor in the gas phase. The tubular structure is biased relative to ground. Plasma having a density is formed and cyclically positioned along the length of the tubular structure. Positive ions of the plasma precursor gas are generated and are deposited on the internal surface forming a coating on the internal surface, wherein the coating exhibits a water contact angle of greater than 120°.

FIELD OF INVENTION

The present disclosure relates to superhydrophobic conformal coatingspresented on the internal surface of relatively long tubular structuresand processes for forming such coatings. In particular, the presentinvention relates to superhydrophobic coatings that mitigate thenucleation, growth and adhesion of hydrocarbon hydrates and inorganicscales on the internal surface of tubular structures.

BACKGROUND

Hydrates and, in particular, clathrate hydrates are understood to becrystalline water-based solids physically resembling ice, in whichmethane and other relatively small hydrocarbons are trapped. Methanehydrate deposits found on and beneath the ocean floor and in certainregions of permafrost constitute the majority of known natural gasreserves on the planet. In addition, hydrates of methane and otherrelatively small hydrocarbons form in producing petroleum wells and gasor oil pipelines. However, hydrate formation within producing wells andpipelines lead to solid plugs of ice with gas trapped within thatocclude product flow when unmitigated.

Approximately 10 to 15% of production costs may be invested in theprevention of hydrate formation using technologies based on chemicaladditives (e.g., methanol, siloxane oligomers, poly-N-vinylpyrrolidone,and aluminum sulfate) and physical methods (e.g., high-frequencyelectromagnetic fields). However, when such preventative methods fail,the removal of a continuous hydrate plug, for example, in an offshorepipeline is costly. Mitigation strategies that are not reliant onchemical additives or external physical methods may offer anextraordinary cost savings to pipeline operations if an intrinsicproperty of the surfaces which contact petroleum, and therefore suchhydrates, can be manipulated to reduce or eliminate the nucleationand/or adhesion of hydrates on such surfaces.

Providing organosilicone functionalization and coatings which decreasethe adhesion of hydrates to these surfaces have been attempted. Thehydrophobization of the metal surface has relied on, for example,reacting iron oxy-hydroxide functional groups present on the surface ofcarbon steels with trimethyl chlorosilane or chlorosiloxane oligomers,as well as the fluorine-substituted analogs of these reactants. Whilethese strategies may result in surfaces that can be classified ashydrophobic relative to water, with water contact angles approaching120°, these levels of hydrophobicity have been considered insufficientto inhibit hydrate nucleation, growth and adhesion on metal surfaces.

SUMMARY

An aspect of the present disclosure relates to a method of depositing aconformal coating. The method includes creating a vacuum within aninterior volume of a tubular structure, wherein the tubular structurealso includes an internal surface. Gas is supplied to the interiorvolume of the tubular structure, wherein the gas includes a plasmaprecursor in the gas phase. The tubular structure is biased relative toground. Plasma is formed that may be cyclically positioned along thelength of the tubular structure. Positive ions of the plasma precursorgas are generated and then may be accelerated to the internal surfaceand deposited on the internal surface forming a coating, wherein thecoating exhibits a water contact angle of greater than 120°.

In a related aspect, the present disclosure relates to a conformalcoating disposed on a tubular structure, comprising a tubular structurehaving an internal surface and an interior volume, and a carbo-siloxanecoating disposed on the internal surface, wherein the coating has awater contact angle of greater than 120°. The coating comprises 50 to 60atomic percent carbon, 20 to 30 atomic percent oxygen, and 15 to 25atomic percent silicon. The coating, at a depth of up to 300 Angstroms,includes the following chemical bond types: (a) Si—O— at a relativeconcentration of 38.5(±12) %;(b) Si—C at a relative concentration of25.6 (±5) %; (c) C—O at a relative concentration of 4.5 (±3) %; and (d)C—C at a relative concentration of 31.4 (±2) %

A further aspect of the present disclosure relates to a system forforming a conformal coating in a tubular structure. The system includesa tubular structure including an internal surface defining an interiorvolume and at least two opposing ends, wherein each opposing endincludes an opening. The system also includes at least two vacuumpumping stations, wherein each vacuum pumping station is coupled to oneof the openings. The system further includes a gas supply port coupledto the interior volume via a tubular electrode of small diameter (i.e.,0.125-0.5 inch OD) at ground (or positive) potential spanning the lengthof the tubular structure and centrally-suspended under tension, throughwhich gas is supplied and exited into the internal volume of tubularstructure at approximately midway of length of tubular structure througha gas diffuser. In addition, as an optional feature, the system mayinclude a plurality of magnetic field coils, wherein each magnetic fieldcoil is arranged around the tubular structure and the magnetic fieldcoils are spaced along the length of the tubular structure. Furthermore,the optional feature of the system includes an arbitrary waveformgenerator electrically connected to the magnetic field coils configuredto impose a variable current to the magnetic field coils and configuredto provide a phase offset between at least two of the magnetic fieldcoils. These optional features of the system may be used to assist thestatic- or dynamic positioning of a plasma as formed inside the tubularstructure.

BRIEF DESCRIPTION OF THE DRAWINGS

The above-mentioned and other features of this disclosure, and themanner of attaining them, may become more apparent and better understoodby reference to the following description of embodiments describedherein taken in conjunction with the accompanying drawings, wherein:

FIG. 1 illustrates a schematic of water contact angle measurement;

FIG. 2 a illustrates a schematic of an exemplary embodiment of a systemfor performing the process of the present disclosure;

FIG. 2 b illustrates a schematic of another exemplary embodiment of thesystem for performing the process of the present disclosure;

FIG. 3 illustrates an exemplary method for performing the process of thepresent disclosure;

FIG. 4 illustrates the shear displacement versus shear force forwater-ice on coated and uncoated structures;

FIG. 5 illustrates the relationship of the water-in-oil contact angleand the shear stress at peak force (MPa) versus the surface roughness ofwater-ice on stainless steel;

FIG. 6 illustrates the shear force versus shear displacement for bothuncoated and coated carbon steel relative to solid asphaltenes; and

FIG. 7 illustrates the Raman spectrum for coatings produces by theprocess and system described herein versus conventional glow-dischargeplasma enhanced chemical vapor deposition.

DETAILED DESCRIPTION

The present disclosure relates to superhydrophobic conformal coatingspresented on the internal surface of relatively long tubular structuresand processes for forming such coatings. In particular, the presentinvention relates to superhydrophobic coatings that mitigate thenucleation, growth and adhesion of hydrocarbon hydrates, organicdeposits, and inorganic scales on the internal surface of tubularstructures.

As noted above, hydrates and, in particular, clathrate hydrates, areunderstood as crystalline water-based solids in which relatively smallhydrocarbons or other low molecular weight compounds are trapped insidehydrogen bonded water molecules. Stated another way, hydrates arecrystalline compounds, which comprise two constituents including hostmolecules (or water molecules) that form a hydrogen-bonded solid latticestructure and guest molecules (including hydrocarbons or otherrelatively low molecular weight compounds) trapped in the hostmolecules. The hydrocarbons are trapped without chemical bonding insidethe three-dimensional lattice structure formed by the hydrogen bondingof the water molecules. Non-limiting examples of hydrocarbons mayinclude methane, ethane, propane, isobutene, 2-methyl butane,methylcyclopentane, methylcyclohexane, cyclooctane and combinationsthereof. Other, non-hydrocarbon hydrates may include carbon dioxide,hydrogen sulfide, nitrogen, chlorine, etc.

Hydrates may form if gas and water are present under the appropriatephase-state conditions. In the case of a pipeline or well, hydrates canform in the bulk flow or on metal surfaces, i.e., surfaces in orbounding the flow. Typically, flow bounding a metal surface is colderthan the bulk flow. The colder temperatures promote hydrate formation onthe metal surfaces, which may provide nucleation sites for the formationof the crystalline lattice. Water can wet the metal, and subsequently,clathrate gas-hydrates can form from gas molecules originally dissolvedin the water phase. Hydrate nucleation may occur at nuclei sizes rangingfrom 5 nm to 30 nm. These nuclei are metastable and may agglomerate intolarger, stable hydrate clusters in the size range of 100 to 2000 nm.

Under conditions of supersaturated gas in the water phase, nucleationproceeds much more actively at the water-metal interface compared withgas-water interfaces away from the metal surface. In particular, thesurface roughness of an unmodified metal substrate, having relativelyhigh surface energy, may play two relatively important roles in theactivity of hydrate nucleation. First, the surface roughness may createareas of water-phase disruption at the surface, lowering the barrier tohydrate formation. Second, the formation of microscopic gas pocketswithin the asperities of the surface enables hydrate growth by creatinga relatively large number of gas-liquid interfaces.

The present disclosure is directed to modifying the surface state of theinternal surfaces of the pipe wall, such that the surfaces exhibitsuperhydrophobicity, reducing the surface energy and wettability ofwater on the pipe wall with respect to water. Hydrophobicity may beunderstood as the repulsion of water from a surface of a given material,this is opposed to hydrophilicity wherein water is attracted to thesurface of a given material. Hydrophobicity may be quantified in termsof contact angle, which as illustrated in FIG. 1, is the angle θ that adrop of water 100 forms relative to a given surface 102. The lower thecontact angle, the more the water is attracted to and wets the surface.The greater the contact angle, the less water wets the surface. Theability of water to “wet” or “wet out” a surface indicates the abilityof the water to flow and cover a surface to maximize the contact areaand the attractive forces between the water and surface. In FIG. 1, thecontact angle may be measured in a mineral oil environment as opposed toan air environment.

Hydrophobic materials may generally be understood as materials havingwater contact angles that are 90° or greater. Superhydrophobic materialsmay be understood herein as materials having a contact angle with waterthat is 120° or greater. Thus, the internal surfaces of the tubularstructures discussed herein are modified to reduce the surface energy ofthe surfaces relative to water therefore increasing the hydrophobicityto contact angles of 120° or greater. As understood herein, themeasurements may be performed in mineral oil. This serves as a referenceenvironment to approximate the environment in an oil-producing well orflow line.

The interior surface of the tubular structures may be modified bycoating the structure with superhydrophobic material which provides aconformal coating. Employing high-resolution X-ray photoelectronspectroscopy (XPS), the relative contribution of different bondingstates comprising the superhydrophobic coating was determined from theaverage XPS-measured states of each element involved in a given bondtype. It has been determined from these quantitative results that thesuperhydrophobic coatings may include, consist essentially of, orconsist of one or more substantially amorphous domains of the followingchemical bond types: (a) Si—O— at a relative concentration of 38.5(±12)%; (b) Si—C at a relative concentration of 25.6 (±5) %,; (c) C—O at arelative concentration of 4.5 (±3) %; and (d) C—C at a relativeconcentration of 31.4 (±2) %. Reference to the feature that the domainsare substantially amorphous may be understood as that situation wherein90% or greater of the domains are amorphous or non-crystalline.

The coatings may include, consist essentially of, or consist of 50 to 60atomic percent carbon, including all values and ranges therein, 20 to 30atomic percent oxygen, including all values and ranges therein, and 15to 25 atomic percent silicon, including all values and ranges therein.The elements are present at a total amount of 100 atomic percent;however, impurities may be present up to 1 atomic percent of the totalcomposition. Preferably, the coatings may be composed of 56 to 57 atomicpercent carbon, including all values and ranges therein, 20 to 26 atomicpercent oxygen, including all values and ranges therein, and 17 to 23atomic percent silicon, including all values and ranges therein. Inaddition, such atomic percent concentrations and the aforementioned bondtypes are such that they are present at the surface and at depths of 100Angstroms, 200 Angstroms and 300 Angstroms into the coating. Thus, theindicated atomic percent concentration of C, O and Si is present in arelatively uniform profile from the surface to a depth of 300 angstroms.

The coatings herein may be preferably formed from silane compoundsproviding C, H, O and Si. Preferably a plasma precursor may includehexamethyldisiloxane (HMDSO) having the formula (CH₃)₃Si—O—Si(CH₃)₃. Itcan be appreciated that the precursors so identified may then form aplasma by themselves or with the assistance of a noble gas such as Ar(preferably) or He, and coat the internal surfaces of the tubularstructure to provide the amorphous domains noted above. Additionally,other preferred precursors useful in forming superhydrophobic coatingson the internal surface of tubular structures, in accordance with theprocess methods described herein, include: (1) hexamethyldisilazane(CH₃)₃Si—N—Si(CH₃)₃; (2) bis-trifluoropropyl tetramethyldisiloxane(CF₃C₂H₄) Si(CH₃)₂—O—Si(CH₃)₂(CF₃C₂H₄); and (3) combinations of HMDSO or(1) or (2) with a volatile fluorocarbon including, but not limited to,perfluoropropane (C₃F₈), hexafluoropropylene oxide (C₃F₆O),perfluorocyclohexane (C₆F₁₂), and hexafluorobenzene (C₆F₆).

Tubular structures on which the superhydrophobic coating are applied maybe understood as structures having a length to diameter (or largestlinear cross-section) ratio of 10:1 or greater, such as 20:1, 30:1,100:1, and up to 1,000:1. The tubular structure may be 10 or more feetin length, including all values and ranges between 10 feet and 100 feet,including all values and increments therein, such as in the range of 10to 50 feet, 20 to 70 feet, etc. The tubular structures may generally becircular in cross-section. However, in other embodiments, the tubularstructures may exhibit rectangular, square, triangular or geometriccross-sections. Additional embodiments may include non-linear (in plane)hollow shapes, such as “S”, bends, and split rings, or helical(out-of-plane) shapes. In these cases, the internalelectrode/gas-manifold at ground (or positive) potential is suspendedwith periodic dielectric supports making contact with the internal wallof the structure, as opposed to an unsupported, though tensioned,electrode for most linear shapes less than or equal to 40-ft in length.Such dielectric supports are translated during the coating process sothat the area of the support making direct contact with the internalwall of the structure is unmasked, thus avoiding the potential forleaving behind periodic uncoated patches or coating variances along thelength of the structure wall at the periodic locations of the saidsupports.

The tubular structures may exhibit an initial average surface roughness(Ra) of less than 10 μm. The tubular structures may be iron based andmay include steel and preferably, stainless steel or carbon steel.

The coatings may be deposited on a structure utilizing amagnetically-assisted plasma enhanced chemical vapor deposition system,an example of which is illustrated in FIG. 2 a. However, it is to beunderstood that magnetic field assistance is not a necessary requirementof the process conditions, but rather may be used as needed to assistthe deposition process by concentrating or translating—eitherdynamically or statically—the plasma formed inside the tubularstructure. The system includes the tubular structure 200, in which thecoating will be deposited, mounted between two high-vacuum pump stations204 a, 204 b on each end 206 a, 206 b of the tubular structure 200,i.e., at each opening of the tubular structure. Accordingly, shouldadditional openings be provided in the tubular structure, additionalhigh-vacuum pumps may be mounted at these openings. Alternatively,additional opening may be sealed to prevent escape of the processinggasses. The high-vacuum pumps 204 a, 204 b are operatively connecteddirectly or indirectly to the tubular structure in such a manner thatgas may be evacuated from the interior volume of the tubular structure.A combination of vacuum pumps may be arranged in the high-vacuum pumpstations. For example, a positive displacement pump to achieve vacuumand a turbomolecular vacuum pump to achieve high-vacuum may be utilized.The high vacuum pump station may achieve a vacuum in the range of 1*10⁻¹Torr to 1*10⁻⁷ Torr.

The tubular structure is electrically isolated from the high-vacuumpumping stations using vacuum compatible insulators 208 a, 208 b.Further, components connected to the vacuum pumping stations 204 areconnected to an electrical ground G. A gas supply system 210 isconnected to a gas inlet port 211 a, 211 b located at distal portions ofthe apparatus 212 a, 212 b. An internal gas manifold 214 (see FIG. 2 b),which also serves as an electrode at ground potential, running along theaxial length L of the tubular structure, including a plurality of gasports 216 may also be provided to distribute the process gasses evenlythroughout the interior volume 220 of the tubular structure 200. The gasports are operatively coupled to the interior volume of the tubularstructure in a manner that a pathway for gas to pass from the storagetanks to the gas ports is provided, filling the interior volume of thetubular structure with the supplied gasses. The gas supply system 210may include storage tanks or devices 222 a, 222 b in which the gas isstored in either a gas or liquid form. The gas supply system 210 mayalso include mass flow controllers 224 a, 224 b for controlling the flowrate of the gas entering the system. The gasses are passed throughsupply lines 226 a, 226 b, 226 c, and 226 d into the gas inlet ports 211a, 211 b. While only two storage tanks and mass flow controllers areillustrated, more than two may be present, such as three, four and evenup to ten. Gasses utilized in the system, process and coatings hereininclude a chemical precursor used alone, or in combination with an inertgas, such as argon. The inert gases maybe supplied to the interiorvolume at a ratio of 1:40 to 10:1 of the inert gas to the plasmaprecursor

A high-voltage, pulsed DC power supply 230 is electrically connected tothe tubular structure 200. The negatively biased, pulsed voltage mayrange from 0.5-10kV, including all values and ranges therein with apulsed frequency ranging from 500-5000 Hz, including all values andranges therein and a pulse width ranging from 1 to 100 μs, including allvalues and ranges therein. The pulsed voltage from the power supply maybe biased negatively relative to the system ground, forming a plasma ina hollow space of the structure. The plasma consists of electrons, ionsand neutrals at various energy states. When the chemical precursors(e.g. HMDSO) is fed into the plasma it may fragment, resulting in theformation of ionized and un-ionized (radicals) fragments of HMDSOmolecules. The negatively biased voltage extracts the positively chargedions from the plasma and accelerates the ions to the internal surface ofthe tubular structure. The acceleration is with relatively high terminalvelocities and relatively high kinetic energy. When energetic ions andradicals of the fragments of the HMDSO molecules deposit on the internalsurface of the tubular structure a coating with the required compositionmay form.

As illustrated in FIG. 2 a, the system preferably includes two or moremagnetic field coils 240 a, 240 b, 240 c, 240 d, i.e., solenoids, areconfigured or arranged around the exterior of the tubular structure.While four coils are illustrated, up to 10 or 20 coils may be provideddepending upon the length of the pipe. Each magnetic field coil mayexhibit the cross-sections shape of a collar and wrap around theperiphery of the tubular structure 200, spaced along the axial length Lof the tubular structure 200. The magnetic field coils are connected toone or more DC, constant-current power supplies 242 a, 242 b producing amagnetic field. The magnetic field may penetrate the wall thickness,depending on the magnetic permeability of the structure. Magnetic fluxdensities in the range of 1 gauss to 2000 gauss, including all valuesand ranges therein, may arise in the hollow region or interior volume220 of the tubular structure, i.e., the plasma region.

An arbitrary wave form generator 244 is also provided and electricallyconnected to the magnetic field coils to impose a variable current toeach field coil 240 combined with a phase offset β between at least two,and up to all of the magnetic field coils, e.g., 0 ° to 180° phaseshift. The plasma density may therefore be cyclically swept orpositioned over the length of the structure. This is reference to thefeature that the plasma may be relatively confined and intensified (e.g.higher relative plasma concentration) at selected locations along thetubular structure length. The swept magnetic flux may therefore form arelatively more uniform plasma within the tubular structures as comparedto that situation where no magnetic flux or a static magnetic flux isprovided. Accordingly, a relatively more uniform coating depositionalong the internal surface of the relatively long hollow structure maynow be achieved as needed.

A process for forming the superhydrophobic coatings is further describedherein with respect to FIG. 3. The process may optionally begin with acleaning step 300, such as sputter cleaning, to remove contaminationincluding organic species or inorganic surface oxides, from the interiorsurfaces of the tubular substrates. Initially, a vacuum is created ordrawn on the interior volume of the tubular member to a pressure in therange of 1*10⁻⁶ Torr to 1*10⁻⁷ Torr, including all values and rangestherein. Argon gas and, optionally hydrogen gas as a reactive gas, isintroduced into the tubular substrate through the gas supply system,such that a pressure in the range of 10 mTorr to ⁵⁰ mTorr, including allvalues and ranges therein, is reached. Other inert gases (e.g., xenon,helium, neon, krypton or combinations thereof) may be used alone or incombination with argon gas. While maintaining the above noted gaspressures, a negatively biased, pulsed voltage ranging from 0.5 to 10kV, including all values and ranges therein, with a pulse frequencyranging from 500 to 5000 Hz, including all values and ranges therein,and a pulse width of 1 to 100 microseconds, including all values andranges therein. Sputter cleaning may occur for a period ranging from 30minutes to 120 minutes, including all values and ranges therein.Positive ions (and/or positive ion-radicals) generated by the plasma areaccelerated towards the negatively biased internal surface of thetubular member with relatively high kinetic energy, resulting in thesputter cleaning of surface contaminants from the surface of thestructure. During this process, negatively charged species, includingelectrons and ion-radicals, accelerate toward the internalelectrode/gas-manifold set at ground potential.

Following the optional plasma sputter cleaning, the chemical precursorgas, such as HMDSO described above, may be supplied alone or co-mixedwith an inert gas, such as argon, and metered into the interior volumeof the tubular structure at a constant flow rate 302. The precursor gasmay be supplied at a flow rate of 1 to 100 sccm, including all valuesand ranges therein, while the inert gas may be supplied at a flow rateof 1 to 200 sccm, including all values and ranges therein. The gasspace, or interior volume, of the tubular structure is maintained at atotal pressure ranging from 10 mTorr to 100 mTorr, including all valuesand ranges therein.

To initiate deposition of the coating onto the internal surface of thetubular structure, a plasma is formed 304 in the interior volume byelectrically exciting the tubular structure through negatively biasingthe tubular structure with a pulsed voltage in the range of 0.5 kV to10kV, including all values and ranges therein, relative to ground at apulse frequency of 500 Hz to 5000 Hz, including all values and rangestherein, and a pulse width ranging from 1 microseconds to 100microseconds, including all values and ranges therein. The depositionperiod 306 may range from 60 minutes to 120 minutes, including allvalues and ranges therein. The resulting coating has a thickness of upto 2.0 micrometers, such as in the range of 0.2 to 2 micrometers,including all values and ranges. In addition, the resulting coating isconformal, i.e., conforming to the surface features of the substrate andexhibiting a deviation in thickness of less than or equal to 50% acrossthe coating. After deposition, a substantially smooth and durablecoating is formed having a chemical and structural composition thatexhibits superhydrophobic surface properties and further inhibits thenucleation, growth, and adhesion of gas hydrates and/or adhesion ofasphaltenes, waxes and inorganic scales on its surface.

The resulting coating compositions are characterized along the linesnoted above. The water contact angle (WCA) of a liquid-water dropresting on the surface of a coated steel substrate immersed in mineraloil is determined to exceed 120° and reaches a value of 155°, includingall values and ranges therein. As surface roughness increases, the watercontact angle increases. Referring again to FIG. 1, the contact angle isdetermined by the angle θ formed between the surface plane 102 of thecoated substrate, parallel with the solid-water interface, and thetangent line 104 at the water-oil interface intersecting the surfaceplane at the solid-water-oil triple point, measured through the waterdrop, wherein the oil is mineral oil. Measurement of the water contactangle may be achieved by methods conventionally practiced, whichinclude, for example, a goniometer coupled with a relatively highresolution camera.

In addition, the resulting compositions exhibit reduced water-ice shearpressures (i.e., shear strengths). Specifically, the water ice shearpressures may be reduced by more than half when utilizing thesuperhydrophobic coatings (having a contact angle of 120° or greater)versus hydrophobic coatings (having a contact angle of less than 120°).The water-ice shear pressure may, for example, be less than or equal to0.2 MPa, such as 0.001 MPa to 0.2 MPa, including all values and rangestherein. The water-ice shear pressure is understood as the amount ofshear stress required to displace a water-ice drop bound to a surface,wherein the water-ice drop and surface are immersed in oil. This may becompared to the water-ice shear pressure of a water-ice drop on anelectro-polished uncoated steel surface of 1.089 MPa. Stated anotherway, the water-ice shear pressure of the coated surface is half, or morethan half, i.e., 1% to 50% of the water-ice shear pressure of uncoatedsurfaces of the same roughness.

Further, the resulting compositions exhibit a reduced shear pressurerelative to hydrocarbon materials such as asphaltene and wax. Shearpressures may also be reduced by half or more than half, i.e., 1% to50%, when utilizing the superhydrophobic coatings, such as less than orequal to 0.010 MPa. Thus, the coating also exhibits oleophobiccharacteristics, which may be understood as a molecule that is repelledfrom oil. In the case of inorganic scale, a shear pressure of less thanor equal to 0.040 MPa may be used to dislodge inorganic scale composedof carbonate salts adhered to its surface, compared with 0.16 MPa forthe bare surface.

The superhydrophobic and oleophobic nature of the coatings are useful inmitigating or inhibiting the nucleation, growth and adhesion ofasphaltenes, waxes, and gas hydrates onto the internal surface of thesteel pipe materials. Such properties bear significant benefits to theoffshore pipeline industry by reducing the frequency in which productflow may be encumbered by stoppages due to gas hydrate, asphaltene orwax occlusions formed in the pipeline.

The compositions of matter of the preferred coatings described hereinare understood to be accessible via the unique combination of thedeposition process, the process conditions, and the preferred chemicalprecursors described herein. This, in turn, provides thesuperhydrophobicity and low surface-adhesion properties necessary forinhibiting the nucleation, growth, and adhesion of gas hydrates and theadhesion of asphaltenes, waxes and inorganic scales on steel substrates.Further, the coating is capable of resisting wear and abrasion.Moreover, the process is amenable to coating the internal surface oflong tubular structures.

EXAMPLES

A number of steel substrates were provided having a variety of averagesurface roughness (Ra) ranging from 7.0 micro-inches to 30 micro-inches.The surface roughness of each substrate was measured using a stylus-typesurface profilometer (Dektak 150, Veeco Instruments, Inc.) andcollecting the surface profile over a scan distance of 1 cm. The averageroughness, Ra, was determined by computing the arithmetic average of theabsolute values of the vertical deviations (hills and valleys) in thesurface profile. Coatings were produced on each sample 0.4 microns and0.8 microns in thickness as described further in Table 1 below.Specifically, each sample was first sputter cleaned and then a coatingwas deposited using HMDSO precursor and Ar gas.

The hydrophobicity of each coating was characterized by measuring thewater contact angle in mineral oil using a goniometer coupled with ahigh-resolution digital camera. As previously illustrated in FIG. 1, thewater contact angle (WCA) was defined as the angle θ formed between thesurface plane 102 of the coated substrate, parallel with the solid-waterinterface, and the tangent line 104 at the water-oil interfaceintersecting the surface plane at the solid-water-oil triple point,wherein the angle passes through the bubble. Digital image processingsoftware (SIMAGIS®, Smart Imaging Technologies, Inc.) was employed toaccurately derive the said WCAs from the captured images of the waterdrop and coated surface. The water contact angles for each sample areprovided below in Table 1. As illustrated, the greater the roughness andthickness of the coating, the greater the water contact angle in oil.However, this effect is expected to reach a maximum, beyond which thecontact angle decreases and the shear pressure, as described below,increases.

TABLE 1 Hydrophobic And Adhesion Characteristics Of The Coatings onCarbon Steel Uncoated Roughness, Coating Ra (micro-inch) Thickness WCAin Oil Water-Ice Shear [micrometer] (micrometer) (deg) Pressure (MPa) 7.0 [0.18] 0.4 112 1.086 12.0 [0.30] 0.8 123 0.445 15.0 [0.38] 0.8 1470.219 30.0 [0.76] 0.8 153 0.063

The adhesion of water-ice characterized by the water-ice shear pressure(i.e., shear strength) was also measured for each coating. Specifically,the water-ice shear pressure was characterized by using an apparatusthat measures adhesion between a water-ice drop and a coating on a steelsubstrate. A description of this apparatus and its application have beenreported elsewhere [Zou, M.; Beckford, S.; Wei, R.; Ellis, C.; Hatton,G.; Miller, M. A., Applied Surface Science 257 (2011) 3786-3792]. Thetesting was performed at 262 K. FIG. 4 illustrates a comparison of theshear force (N) utilized to break the adhesion between bare,electropolished stainless steel A having a surface roughness of 5.0micro-inches (0.13 micrometer) and stainless steel having the sameroughness and a coating having a thickness of 0.8 microns B. As can beseen in FIG. 4, the shear stress required to displace a drop ofwater-ice bound to the coated surface was determined to reach a value aslow as 0.0363 MPa at 262 K, whereas the electropolished uncoated steelsurface requires a shear stress approximately one order or magnitudehigher to dislodge a water-ice drop of the same volume bound to itssurface at a similar temperature. Table 1 lists the water-ice shearpressures for each coating and surface roughness on carbon steelsubstrates. In addition, FIG. 5 illustrates the relationship of thewater-in-oil contact angle θ and the shear stress at peak force (MPa) Sversus the surface roughness.

The adhesion of solidified asphaltenes and wax was also determined usingthe measurement apparatus described above at 262K. In this example, thesubstrates were formed from carbon steel having a surface roughness of30 micro-inch (0.76 micrometer). FIG. 6 illustrates the shear forceversus shear displacement for both the bare carbon steel sample A andthe coated carbon steel sample B. The peak shear-stress required todislodge a drop of solid asphaltene from the surface of uncoated carbonsteel in this example was 30.2 kPa, compared with 9.86 kPa for thecoated steel.

The elemental composition and bonding states of matter of the preferredcoatings derived from the conditions of the process may be characterizedwith respect to depth or thickness of the coating via X-rayphotoelectron spectroscopy (XPS) also known as electron spectroscopy forchemical analysis (ESCA). XPS (or ESCA) may be understood as anelemental analysis technique for detecting elements with the exceptionof hydrogen and helium, having a nominal detection limit ofapproximately 0.1 atomic %. High-resolution XPS may also provideinformation regarding the bonding states of the elements in question.The elemental composition of the coatings formed herein is described inTable 2. Tables 3 through 5 provide the bonding state of the coatings.

TABLE 2 Relative Elemental Composition By Atomic Percent Of ThePreferred Coating As A Function Of Depth Depth (Å) From Surface CarbonOxygen Silicon Surface 57 26 17 100 56 22 22 200 57 21 22 300 57 20 23

TABLE 3 Relative Concentration Of Bonding States For Carbon 1s HighResolution Region Of The XPS As A Function Of Depth Depth (Å) C—Si C—C,C—H C—O O—C═O Surface 33 61 4.8 1.5 100 43 55 2.6 — 200 44 53 3.0 — 30042 54 3.4 —

TABLE 4 Relative Concentration Of Bonding State For Oxygen 1s HighResolution Region Of The XPS As A Function Of Depth Depth (Å) O—Si O—CSurface 86 14 100 83 17 200 90 10 300 91 9.1

TABLE 5 Relative Concentration Of Bonding State For Silicon 2p HighResolution Region Of The XPS As A Function Of Depth Depth (Å) Si—C*Si—O* Surface 38 62 100 55 45 200 55 45 300 55 45 *Combination of Si2p_(3/2) and Si 2p_(1/2) peaks

In addition, the coatings were examined utilizing Raman Spectroscopy toidentify the unique molecular-structural characteristics of obtainedusing the processes claimed herein. A comparison was performed betweenthe Raman spectrum of the coating and process described herein to thatof conventional glow-discharge plasma enhanced chemical vapor deposition(PECVD). In both cases, HMDSO precursor was used in combination with Arprocess gas. The spectra are illustrated in FIG. 7, wherein the spectrumexhibited by the present process is indicated as I and the spectrumexhibited by the conventional process is indicated as II.

As seen in the graph, the process of the described herein yields acoating that contains a substantially larger proportion of —Si—O—Si—structural groups than the conventional process. This is indicated bythe symmetric —Si—O—Si— stretching vibrations in a broad region of thespectrum near 500 cm⁻¹, at point A, which is markedly depressed in thecoating prepared by glow discharge. The broad spectral region near 793cm⁻¹, point B, indicates a proportionally large contribution of terminaldi- and trimethyl silanes [—OSi(CH₃)_(x)], which emerge in the Ramanspectrum as asymmetric Si—C stretching vibrations. Lastly, the spectralregion between 1309 cm⁻¹ and 1350 cm⁻¹, point C, can be assigned to—CH2— scissoring modes associated with the formation of carbosilanes[Si—CH₂—Si] in the coating. The relative contribution of this structuralfeature is again greater in the coating described herein than in theconventional coating.

While particular embodiments have been described, it should beunderstood that various changes, adaptations and modifications can bemade therein without departing from the spirit of the invention and thescope of the appended claims. The scope of the invention should,therefore, be determined not with reference to the above description,but instead should be determined with reference to the appended claimsalong with their full scope of equivalents. Furthermore, it should beunderstood that the appended claims do not necessarily comprise thebroadest scope of the invention which the applicant is entitled toclaim, or the only manner(s) in which the invention may be claimed, orthat all recited features are necessary.

What is claimed is:
 1. A method of depositing a conformal coatingcomprising: creating a vacuum within an interior volume of a tubularstructure, wherein said tubular structure includes an iron basedinternal surface and said interior volume; introducing an inert gas andoptionally hydrogen and sputter cleaning said iron based internalsurface; directly following said sputter cleaning supplying gas to saidinterior volume of said tubular structure, wherein said gas includes aplasma precursor in the gas phase; biasing said tubular structurerelative to ground; forming a plasma having a density and cyclicallypositioning said plasma density along the length of said tubularstructure; generating positive ions of said plasma precursor gas whichare deposited on said internal surface; and forming a coating having asurface on said internal surface, wherein said coating exhibits a watercontact angle of greater than 120° and said coating comprises 50 to 60atomic percent carbon, 20 to 30 atomic percent oxygen and 15-25 atomicpercent silicon, and said atomic percents of carbon, oxygen and siliconare present at the surface of said coating and at a depth of up to 300Angstroms into said coating and wherein said coating mitigates thenucleation, growth and adhesion of asphaltenes waxes and gas hydrates.2. The method of claim 1, further comprising arranging at least twomagnetic field coils externally around said tubular structure andforming a magnetic field having a density in the range of 1 gauss to2,000 gauss in the hollow region of the tubular structure.
 3. The methodof claim 2, wherein a variable current is imposed to each magnetic fieldcoil with a phase offset between each magnetic field coil to cyclicallysweep said plasma density over the length of the tubular structure. 4.The method of claim 1, wherein said plasma precursor compriseshexamethyldisiloxane.
 5. The method of claim 4, further comprisingsupplying an inert gas, wherein said gases are supplied to said interiorvolume at a ratio of 1:40 to 10:1 of said inert gas to said plasmaprecursor.
 6. The method of claim 1, further comprising sputter cleaningsaid tubular structure.
 7. The method of claim 1, wherein said coatingcomprises 56 to 57 atomic percent carbon, 20 to 26 atomic percent oxygenand 17 to 23 atomic percent silicon.
 8. The method of claim 1, whereinsaid coating, at a depth of up to 300 Angstroms, comprises the followingbond types: (a) Si—O at a relative concentration of 38.5(±12) %; (b)Si—C at a relative concentration of 25.6(±5) %; (c) C—O at a relativeconcentration of 4.5(±3) %; and (d) C—C at a relative concentration of31.4(±2) %.
 9. The method of claim 1 wherein said coating has a watercontact angle of greater than 120° to 155° .